This invention relates to a magnetic-focus electrostatic-deflection system for an image pickup tube and especially to improvements in its characteristics.
As one of the types of focus and deflection schemes for image pickup tubes, a so-called magnetic-focus electrostatic-deflection system (hereinafter simply referred to as an MS system) has been known which utilizes a magnetic field for beam focusing and an electrostatic field for beam deflection. The structure of this system for an image pickup tube is described in detail by, for example, in U.S. Pat. No. 3,319,110, May 9, 1967 and U.S. Pat. No. 3,796,910, May 12, 1974.
A specific construction of a prior art MS image pickup tube is illustrated in FIG. 1. Referring to FIG. 1, electrostatic deflection electrodes 1 in the form of quadrupole electrodes are adapted to produce uniform deflection fields in horizontal and vertical directions. The electrostatic deflection electrodes 1 are in close contact with the inner surface of a tube 2. Near one end (rear) of the image pickup tube, an electron gun 3 for generation of an electron beam is placed and at the other end (front), a photoconductive target 5 is formed on a faceplate and a mesh electrode 4 is supported in spaced relationship to the photoconductive target 5. These components are all housed in the tube 2. A focus coil 7 surrounds the tube 2 and generates a magnetic field for focusing the electron beam.
The unrolled deflection electrodes seen from the inside of the electrodes are shown in FIG. 2. Zigzag-shaped electrodes were invented by K. Schlesinger (U.S. Pat. No. 2,681,426, June 15, 1954) and were referred to as the curved arrow pattern yoke. The shapes of the electrodes are sometimes modified by twisting them about the axis of the tube to reduce raster distortion and improve deflection sensitivity (for example, see FIG. 3 of U.S. Pat. No. 3,666,985, May 30, 1972).
The twist angle .omega. is indicated in the FIG. 2. The pitch of the deflection electrodes is L.sub.0 and the number of the repetitions is N. The total length of the deflection electrodes is NL.sub.0. The positive Z direction is taken to be the direction in which undeflected electrons travel. The .theta. direction is the circular direction around the axis Z of the tube. (V.sup.+, V.sup.-) and (H.sup.+, H.sup.-) are the vertical and horizontal deflection electrodes respectively. This MS image pickup tube is said to be advantageous in that theoretically, uniform resolution can be obtained over the beam scanning area and distortion can be minimized.
In practical image pickup tubes, however, the electrode of the electron gun 3 is so constructed that a region extending across an interval L.sub.1 between an aperture 6 and the fore end of the electron gun electrode 3 is shielded from the electrostatic deflection field generated by the electrostatic deflection electrode 1, as shown in FIG. 1. Consequently the electrostatic deflection field is almost zero in the region. The electrostatic deflection field is also almost zero in a region extending across an interval L.sub.3 between the fore end of the electrostatic deflect-on electrode 1 and the mesh electrode 4. Further, in a region extending across an interval L.sub.4 between the mesh electrode 4 and the photoconductive target 5, there occurs only a strong electrostatic deceleration field E.sub.Z and no electrostatic deflection field exists.
On the other hand, because of a finite length of the focus coil 7, a magnetic field produced thereby is not constant and uniform on the tube axis but is distributed as shown in FIG. 3 to approximate a Gaussian distribution.
Hence, the MS image pickup tube exhibits deflection aberrations and landing error due to the lack of uniformity of the electromagnetic field.
Accordingly, the practical image pickup tube encounters a problem that when the electron beam is deflected, the beam spot size becomes larger and raster distortion exists.